Conventional gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. In turbofan engines having multistage compressors, stator vanes are placed at the entrance and exit of the compressor section and between adjacent compressor stages in order to direct the air flow to each successive compressor stage. Variable stator vanes, whose pitch can be adjusted relative to the axis of the compressor, are able to enhance engine performance by altering the air flow through the compressor section in response to the changing requirements of the gas turbine engine.
A high pressure compressor variable stator vane assembly 10 is shown in FIGS. 1 and 2. The assembly 10 includes a stator vane 12 mounted within an opening 38 in a casing 22 of a gas turbine engine. As known in the art, in order to alter the pitch of the vane airfoil relative to the axis of the compressor, the stator vane 12 is designed to rotate within the opening 38 of the casing 22. While various configurations are possible for variable stator vane assemblies, the vane 12 shown in FIGS. 1 and 2 has a radially extending flange 30 from which an annular-shaped portion extends axially to define a pair of seats 28 (unless otherwise noted, radial and axial directions referred to are with reference to the centerline of the vane assembly 10, and not the radial and axial directions of the engine in which the assembly 10 will be installed). A trunnion 34 also extends axially relative to the flange 30, and with the seats 28 projects through the opening 38 as seen in FIG. 2. The vane 12 is secured to the casing 22 with a nut 20 that also secures a spacer 14, sleeve 16 and lever arm 18 to the trunnion 34. Rotation of the vane 12 within the opening 38 is caused by actuation hardware (not shown) attached to the lever arm 18.
During engine operation, an overturning moment is created by the gas loads on the vane airfoil, generating reaction forces represented by the arrows "F" in FIG. 2. As a result, rotation of the vane 12 relative to the casing 22 requires a seal assembly that minimizes wear, friction, and compressor air leakage while subjected to the reaction forces F, as well as withstands the hostile thermal and chemical environment of a gas turbine engine. In FIGS. 1 and 2, a seal assembly is shown as consisting of a bushing 24 and washer 26 between the spacer 14 and flange 30 on opposite sides of the casing 22. The bushing 24 and washer 26 are preferably molded from composite materials, such as polyimide resin with glass and TEFLON.RTM. fibers, in order to be environmentally compatible with the engine environment, as well as provide suitable low-friction bearing surfaces that enable the vane 12 to rotate at acceptable torque levels.
The ability to minimize radial air leakage from the compressor through the opening 38 of the casing 22 is an important function of the bushing 24 and washer 26. As can be appreciated from FIG. 2, the dual functions of the bushing 24 and washer 26 to form an air seal yet enable rotation of the vane 12 are determined by the clearance (radial relative to the axis of the compressor) through the bushing 24 and washer 26 between the flange 30 of the vane 12 and an outer annular surface 36 of the spacer 14. To minimize compressor air leakage, the vane 12 and spacer 14 must be assembled to the casing 22 so that the minimum possible clearance is achieved. However, an excessively small clearance results in high forces being required to turn the vane 12, which can overstress the actuation hardware and, in the extreme situation, could completely prevent actuation of the vane 12, leading to compressor stall. On the other hand, an excessive clearance will not only permit excessive air leakage from the compressor, but will also permit the reaction forces on the vane 12 to cause excessive tilting of the vane assembly 10. If this occurs, the reaction forces F are more concentrated in the bushing 24 and washer 26 and, in combination with higher leakage through the seal assembly, causes more rapid deterioration of the bushing 24 and washer 26.
From FIG. 2, it can be seen that the clearance through the bushing 24 and washer 26 is determined by the axial offset dimension "D" between the annular surface 36 and a pair of shoulder 32 of the spacer 14. When the vane 12 and spacer 14 are properly assembled, each of the shoulders 32 abuts one of the seats 28 of the vane 12 as shown in FIG. 2. Increasing the offset dimension D reduces the clearance through the vane 12 and spacer 14 but increases the actuation torque required to rotate the vane 12, while decreasing the offset dimensions D increases the clearance but decreases the actuation torque.
In the art, variable stator vane assemblies of the type shown in FIGS. 1 and 2 have been assembled to attain a torque level within an acceptable range for the actuation hardware. Because it has been assumed that a close relationship exists between the offset dimension D and the torque required to rotate the vane 12, spacers 14 with incrementally different offset dimensions D have been purposely manufactured to allow adjustment of both the actuation torque and radial clearance by substituting spacers 14. After assembly, if the torque required to rotate a vane is outside preestablished torque limits, the nut 20, lever arm 18, sleeve 16 and spacer 14 are removed and the spacer 14 replaced with another having a different offset dimension D. For example, if the actuation torque was too high, a spacer 14 with a smaller offset dimension D was installed, while a spacer 14 with a greater offset dimension D is installed if an unacceptably low torque is measured. Once reassembled, torque is again remeasured and the process repeated if the torque remains outside the established limits.
Notwithstanding the above, further investigations have shown that the torque required to rotate the stator 12 is surprisingly relatively independent of the spacer 14 installed, and that torque is not a reliable indication of the radial clearance between the vane 12, spacer 14 and casing 22. Instead, actuation torque has been found to be primarily determined by irregularities and interferences of the bushing 24 and washer 26 after they have been compressed by the load generated between the flange 30 and spacer 14 by the nut 20. These irregularities and interferences are not predictable particularly since, while molded to tight tolerances, the composite bushing 24 and washer 26 can distort in the free state due to residual stresses, etc.
In view of the above, it can be seen that it would be desirable if a method were available for assembling a variable vane stator assembly to more consistently achieve minimum radial clearances without exceeding acceptable actuation torque levels.